Cathodes can emit electrons by photoemission, thermionic emission, and field emission, or as the result of negative electron affinity. A field-emission cathode (or field emitter) emits electrons when subjected to an electric field of sufficient strength. The electric field is created by applying a suitable voltage between the cathode and an electrode, typically referred to as the anode or gate electrode, situated a short distance away from the cathode.
Chason, U.S. Pat. 5,019,003, discloses a flat-panel display that utilizes a field emitter in which a group of electron-emissive particles are distributed across the top of a substrate. A three-layer sandwich consisting of a lower dielectric layer, an electrically conductive gate electrode layer, and an upper dielectric layer is situated over the substrate and electron-emissive particles. Openings extend through the three layers down to the substrate to expose a group of the electron-emissive particles within each opening. The electron-emissive particles serve as cathode elements.
A viewing-screen structure overlies the field emitter. The screen structure consists of a transparent screen, a patterned anode lying along the bottom of the screen, and luminescent material situated along the bottom of the anode directly above the top of the field emitter. The pattern of the anode corresponds to picture elements ("pixels") of the display.
Jaskie et al, U.S. Pat. 5,278,475, and Kane et al, U.S. Pat. 5,252,833, disclose field-emission flat-panel displays similar to that of Chason. In the field-emitter portions of the displays of Jaskie et al and Kane et al, openings extend through a gate electrode layer and an underlying dielectric layer to expose diamond particles formed on conductive/semiconductive paths situated on a substrate. The diamond particles provide electrons. An anode viewing-screen structure configured in basically the same way as that of Chason overlies the field emitter at a short distance above the gate electrode.
The flat-panel displays of Chason, Jaskie et al, and Kane et al generally operate in the following way. When the gate electrode is placed at a suitable voltage condition, electrons extracted from the electron-emissive particles at the bottom of one of the openings in the field emitter move generally toward the luminescent material of the anode viewing-screen structure. Upon being struck by impinging electrons, the luminescent material emits light which is visible at the exterior surface of the transparent plate. By appropriately controlling the voltage condition of the gate electrode, only electrons from electron-emissive particles in selected ones of the openings strike the luminescent material. A corresponding image is thereby produced on the viewing screen.
The gate electrode in a flat-panel CRT display can be used (a) to directly extract electrons from the electron-emissive elements or (b) to control the movement of electrons extracted by the anode. The gate electrode typically serves as an electron extractor in large-area light-weight flat-panel displays where internal supports are placed between the cathode and anode structures to withstand external pressures exerted on the display and thereby achieve a substantially constant cathode-to-anode spacing across the viewing area. The presence of the internal supports commonly limits the applied anode-to-cathode electric field to values less than that needed to adequately extract electrons from the electron-emissive elements.
Only part of the electrons moving towards the anode strike pixels that the electrons are intended to hit. Some of the electrons strike other parts of the flat-panel structure. Display performance is thereby degraded. In flat-panel CRT displays where the gate electrode functions as the electron extractor, this problem is of particular concern because the voltage on the gate electrode often causes the electrons to diverge from trajectories that end at desired parts of the luminescent material in the anode structure.
Specifically, some of the emitted electrons strike the gate layer and generate a leakage current. Other electrons strike the dielectric layer below the gate layer and cause charge to build up on the dielectric, thereby distorting the local electric field to which the electrons are subjected. It would be desirable to have a field-emission structure in which more of the electrons strike desired anode areas.